The present invention relates to a thermal printing head which is adapted to improve printing resolution and to prevent light printing of areas which are not addressed.
In conventional thermal printing devices, a thermal printhead is provided in which a plurality of dots, i.e., heating resistor elements, are formed in a horizontal row of a dot row resistor material. In such a thermal head, current is supplied to each dot to generate heat, thereby darkening or coloring heat-sensitive paper for thermal printing. A diode matrix is used to address each particular dot in a group of consecutive dots and to prevent current from flowing to dots which have not been selected to print. The group of consecutive dots is chosen to correspond to the width of a column which is printed on the heat-sensitive paper.
In such a conventional printhead, current from activated dots in a particular group may leak to an adjacent group of consecutive dots thereby causing a light printing of adjacent dots which have not been addressed. If light printing of unselected dots occurs, the alpha-numeric character or graphics will have a degraded resolution.
FIG. 1 illustrates a conventional solution to the problem of light printing by physically isolating adjacent groups. FIG. 1 (not drawn to scale) illustrates a partial schematic view of a portion of a conventional thermal printhead. In the figure, a row of dot resistor material 16 is divided by conductive inner fingers 100-112 into individual dot resistors. The dot resistors are arranged in a predetermined group 500 of consecutive dots. Lead wires 300-313 are provided to supply electricity to the individual dot resistors. A second group 501 is partially illustrated having conductive fingers 200-208 and lead wires 400-408 which correspond to fingers 100-108 and lead wires 300-308, respectively, of group 500. It is known in the art that the number of groups may be varied to coincide with the width of the paper.
In the conventional printhead, the inner fingers 100-112 and 200-208 and lead wire portions 300-313 and 400-408 are constructed of an electrically conductive material, such as gold. The fingers and lead wires may be formed on a glazed alumina ceramic substrate 15, or the like, by metallization and wet etching techniques, or screen printing techniques, as are known in the art. The dot row resistor material 16 may be formed of R.sub.u O.sub.2 -based thick film resistive ink such as DUPONT 1700 series and may be mixed in a glass frit and then screened across the inner fingers 100-112 and 200-208, these techniques all being well-known to those having ordinary skill in this art. A protective glass layer (not shown) can be screened and then fixed (e.g., by firing) over the thermal printhead or the dot resistor portion thereof. A flexible cable 51 connects the thermal printhead to a printhead controller (not shown). A housing 53 is connected with screws 52 to the substrate 15 for protection of the thermal printhead. Further, arrow 1000 represents the direction of movement of the thermal paper (not shown). An example of a conventional printhead as just described is the GULTON 01160 diode-matrix printhead.
In the conventional printhead, a group is physically separated from an adjacent group by cutting the dot resistor material 16 with a diamond saw or the like. The physical separation between groups can be defined as an inter-group gap 119. As a result, leakage current is unable to flow across the inter-group gap 119 from one group 500 to another 501. However, with a thermal head of this kind, the cutting of the row of dot resistor material 16 results in a space formed in the pattern printed on the thermal paper which is proportional to the size of the inter-group gap. Thus, the conventional thermal head is not suitable for continuous bar code or graphics printing.
Other examples of conventional printheads are: U.S. Pat. Nos. 4,623,903 to Hashimoto; 4,203,119 to Naguib et al.; 4,559,542 to Mita; and 4,034,187 to Tomioka et al. The complete disclosure of each of these patents is incorporated herein by reference.